The present invention relates to an extrusion molding process, and more particularly to such an extrusion molding process in which the extruded tubular molding from an extruding die is clamped by two clamping devices at different elevations and downwardly stretched, and two symmetrical halves of a molding die are closed on the clamped molding between the clamping devices, causing the clamped molding to be compressed and molded into a finished product.
According to a conventional method of fabricating hollow plastic products through an extrusion molding process, plastic material is extruded into a molding through an extending die and then seized by clamping means and retained in a die cavity of a molding die, and then a cutting tool is moved to cut off the molding from the extruding die, and then a nozzle is inserted into the molding and air is blown into the molding in the die cavity of the molding die, causing the molding to be expanded and molded into a finished product. After molding, the molding die is lowered and then opened. After the finished product is removed from the molding die, the molding die is returned to its former position for a next molding cycle. This extrusion molding method has numerous drawbacks outlined hereinafter:
1. Nozzle and blower means must be provided so that air can be blown into the extruded molding, causing it to expand in the die cavity of the molding die.
2. When thermoplastic material is used and extruded into a molding through the extruding die and the extruded molding is seized by clamping means, the nozzle must be immediately inserted into the extruded molding so that the extruded molding can be swollen with air, avoiding its two opposite side walls from adhering together. This blow molding procedure is not suitable for making a hollow product having a short pitch between its two opposite side walls.
3. If the peripheral wall thickness of the extruded molding is not uniform, the extruded molding will bias from the course, causing itself unable to fall to the accurate position for catching by clamping means. Even if the extruded molding is seized by clamping means and put in the die cavity of the molding die for molding, the ununiform wall thickness may cause the side walls of the finished product to be partially adhered together after the finished product is removed from the molding die.
The present invention has been accomplished to provide an extrusion molding process which eliminates the aforesaid drawbacks. According to the extrusion molding process of the present invention, when a molding is extruded from an extruding die, one clamping device of a molding die unit is clamped on the bottom end of the extruded molding and lowered to stretch the extruded molding downwards, then a second clamping device of the molding die unit is clamped on the upper end of the extruded molding, causing an air-tight space to be formed in the clamped molding, and then two symmetrical halves of a molding die are moved toward each other between the two clamping devices and then closed on the clamped molding, causing the molding to be compressed in the die cavity of the molding die. When the molding is compressed, its inside pressure is relatively increased, thereby causing the wall of the molding to be closely attached to the surface of the die cavity of the molding die. When the molding die is opened after molding, the clamping devices are released, and the molding die unit is lifted to its former position for a next molding cycle. According to another aspect of the present invention, the extruding die comprises a conical core, an air nozzle mounted in the conical core, a ring-shaped mold plate mounted around the conical core and defining a gap around the conical core through which gap fed material is extruded into an extruded molding, and adjustment screws controlled to adjust the size of the gap between the conical core and the ring-shaped mold plate subject to the depth of the die cavity of the molding die. According to still another aspect of the present invention, the die cavity of the molding die has a raised recess which makes a particular part of the wall of the molded product thinner, so that the thin wall portion of the molded product can be forced to break by the inside pressure of the molded product upon opening of the molding die, for permitting the gas inside of the molded product to escape.